1. Field of the Invention
The present invention relates to charge pumps, and particularly to a charge pump for generating arbitrary voltage levels.
2. Description of Related Art
In an electronic circuit, power source voltages of different levels are often required; therefore, a charge pump circuit for generating power source voltages of different levels with the present power source voltage is usually equipped therein.
FIG. 1A is a circuit diagram of a conventional charge pump. FIG. 1B is a timing sequence diagram of control signals for illustrating the charge pump according to FIG. 1A. Referring to FIGS. 1A and 1B together, the charge pump has five transistors; the control signals ph1 and ph2 are opposite in phase, wherein it is Vdd when at high voltage level and it is zero when at low voltage level.
During the charging period (CP) of a capacitor C1, the control signals ph1 and ph2 are respectively at low voltage level and high voltage level. Therefore, the transistors M1 and M3 are turned off while the transistors M0, M2 and M4 are turned on. The voltage at the terminal of the capacitor C1 which is coupled to the control signal ph1 is zero. The power source voltage Vdd charges the capacitor C1 via the transistor M0 until the voltage difference between the two terminals is Vdd−Vt; wherein, Vt is a threshold voltage of the transistors M0 to M4.
During the pumping period (PP) of the capacitor C1, the control signals ph1 and ph2 are respectively at high voltage level and low voltage level, therefore the transistors M1 and M3 are turned on while the transistors M0, M2 and M4 are turned off. The voltage at the terminal of the capacitor C1 which is coupled to the control signal ph1 is Vdd. Because a charge of voltage difference of Vdd−Vt has been stored during the charging period CP, the voltage of another terminal of the capacitor C1 will raise up to 2Vdd−Vt. Also, the voltage of a terminal of a capacitor C2 which is coupled to the control signal ph2 is zero, therefore the voltage 2Vdd−Vt of another terminal of the capacitor C1 charges the capacitor C2 via the transistor M1 until the voltage difference between the two terminals of the capacitor C2 is 2Vdd−2Vt.
Then, by analyzing the voltage variation according to the foregoing method, it can be known that the output voltage Vo from the charge pump is 5×(Vdd−Vt). Similarly, if the charge pump has N transistors, the output voltage is N×(Vdd−Vt). It can be known from the formula of the output voltage Vo that the charge pump can only generate some power source voltages having certain voltage levels.